I agree, generally, though I see 2 cases where seminars may be worthwhile (to the students, not the presenter).
One, is when the attendee is advanced in the same or related art. With their knowledge and experience, they may be able to take some concepts / techniques and put them in a helpful context and later, on their own, develop them into effective application.
Two, when the attendee is less advanced, they probably won't take much of anything specific that's usable, but the seminar may get them excited enough to later seek out true instruction.
For everyone else, maybe some good snacks, socializing, and a certificate of participation may be worth the money.